I want to build an Adalight (first 50LEDs later about 100) is it possible to power the LEDs with an ATX power supply (its 350W and powering my HTPC at the same time).Then I would just put the Adruino into the case and could save an external power supply.

Sure is! If this is also your PC power supply, you should be fine pulling a few amps @ 5V from a spare drive power connector. To use a standalone ATX supply, there are a number of tutorials on Instructables.com...on most modern supplies, it's a simple matter of a jumper between two pins on the motherboard socket.

With that many LEDs, I'd recommend "fanning out" the power rather than driving the strand from one end. With 50 LEDs, it's simple enough to just connect both ends to the power supply. When you upgrade later, the last page of the Adalight tutorial has some advice on powering longer strands.

You might be getting close to the current limit of the Molex connector with 100 LEDs. If there's more than one free connector available, split the strand power across two.

I'm also planning to use the ATX power supply in my HTPC to power my 100 LED Adalight. I just made a sketch on how i plan to wire everything. I would apprechiate if someone could hav a look on it. I will be using two Molex connectors in order not to exceed the maximum current. do these Connectors have to be on different strands from the power supply? And if yes do i have to disconnect the red dashed line in the image? As far as i can tell within the powersupply itself there is only one 5V rail but i'm not a 100% sure.